Citrulline

Citrulline exists primarily in the liver, where it is heavily involved in the urea cycle to detoxify and excrete ammonia. This unusual amino acid is formed in the urea cycle by the addition of carbon dioxide and ammonia to ornithine. Next, it is combined with aspartic acid to form arginosuccinic acid, which later is metabolized into the amino acid arginine. Citrulline is not a component of any major proteins or enzymes.